When crude oil is initially recovered from an oil-bearing earth formation, the oil is forced from the formation into a producing well under the influence of gas pressure and other pressures present in the formation. The stored energy in the reservoir dissipates as oil production progresses and eventually becomes insufficient to force the oil to the producing well. It is well known in the petroleum industry that a relatively small fraction of the oil in subterranean oil reservoirs is recovered during this primary stage of production. Some reservoirs, such as those containing highly viscous crude, retain 90 percent or more of the oil originally in place after primary production is completed. Oil recovery is frequently limited by capillary forces that impede the flow of viscous oil through interstitial spaces in the oil-bearing formation.
Numerous methods have been proposed for recovering additional oil that remains the in oil-bearing formations following primary production. These secondary recovery techniques generally involve the expenditure of energy to supplement the expulsive forces and/or to reduce the retentive forces acting on the residual oil. A summary of secondary recovery techniques may be found in U.S. Pat. No. 3,782,465, the entire disclosure of which is incorporated by reference herein.
One secondary recovery technique for promoting oil recovery involves the application of electric current through an oil body to increase oil mobility and facilitate transport to a recovery well. Typically, one or more pairs of electrodes are inserted within the underground formation at spaced-apart locations. A voltage drop is established between the electrodes to create an electric field through the oil formation. In some processes, electric current is applied to raise the temperature of the oil formation and thereby lower the viscosity of the oil to facilitate removal. Other methods use electric current to move the oil towards a recovery well by electroosmosis. In electroosmosis, dissolved electrolytes and suspended charged particles in the oil migrate toward a cathode, carrying oil molecules with them. These methods typically use a DC potential source to generate an electrical field across the oil-bearing formation.
Oil recovery methods that utilize electrodes frequently encounter problems affecting the quantity and quality of the recovered oil. Systems using straight DC voltage typically operate under high voltages and currents. In addition, systems using DC current consume relatively large amounts of electricity with corresponding large energy costs.